The present application generally relates to a dual chamber cardiac pulse generator. The present invention more particularly relates to an implantable dual chamber cardiac pulse generator which provides essentially periodic pacemaker mediated tachycardia (PMT) control.
Implantable pulse generators, commonly known as pacemakers, are well known in the art. Early pacemakers were single chamber pacemakers that only paced the ventricles in a trigger mode. They did not sense any cardiac activity and paced the ventricles at a predetermined, fixed rate.
Later single chamber pacemakers both sensed ventricular activity and paced the ventricles. The ventricular sensing allowed the pacemaker to inhibit pacing when a spontaneous ventricular activation (R wave) was sensed within an escape interval corresponding to a fixed pacing rate. Such pacing is referred to as demand pacing since the heart is paced only when necessary. This pacing modality is referred to in the art as VVI pacing.
As the pacemaker art advanced, dual chamber pacemakers were made available. The first dual chamber pacemakers sensed in both the atria and ventricles and paced only the ventricles. These dual chamber pacemakers, known as VDD pacemakers, were primarily for heart block patients who suffered from lack of conduction between the atria and ventricles. Their purpose was to simulate normal atrial-ventricular synchrony in heart block patients by coupling ventricular response to atrial activity. When an atrial activation (P wave) was sensed, it started the timing of an AV delay. At the end of the AV delay, the ventricles were paced. The most significant benefit of the foregoing was that when the atrial rate increased due to exercise or some other cause of increased metabolic demand, the ventricular rate would similarly increase so that the hemodynamic output of the heart would satisfy the metabolic demand. Such pacemakers could also function in a demand mode supported by ventricular sensing.
Atrial pacing was later added to the capabilities of dual chamber pacemakers. These pacemakers are referred to in the art as DDD pacemakers. They not only assist heart block patients by coupling the atria and ventricles, but further promote atrial function in sick-sinus syndrome patients whose atria generally do not function properly on their own.
Pacemakers operating in the DDD or VDD modes can, under certain circumstances, sustain a dangerous tachycardia condition. This condition, known as pacemaker mediated tachycardia (PMT) is an operational pacing state wherein the pacer erroneously stimulates the ventricle of a heart at a dangerously high rate for sustained periods of time.
Pacemaker mediated tachycardia is initiated when a ventricular activation occurs at a time during which the connective tissue between the atria and ventricles can transmit retrograde electrical signals from the ventricle to the atrium. The conduction of the ventricular signal to the atrium provides a spurious stimulation electrical signal in the atrium that appears to the pacer to be a normal atrial activation. The pacer senses this spurious retrograde atrial signal and then paces the ventricle at a predetermined AV delay time period following the sensed atrial signal. The paced ventricular signal is subsequently conducted retrograde to the atrium where it is again erroneously detected by the pacemaker as a natural atrial activation. The pacemaker therefore continues to pace the ventricle at a relatively high rate defined by the sum of the programmed AV delay time period and the retrograde conduction time between the ventricles and atria. This high rate is sustained indefinitely by the pacemaker, because retrograde conduction ensures that the pacemaker detects what appear to be high rate atrial events and tracks the spurious atrial events by generating a corresponding high rate ventricular paced stimulus. This pacemaker mediated tachycardia condition over-stimulates the heart at potential danger to the patient.
In order to preclude retrograde conducted ventricular signals from being treated by the dual chamber pacemaker as atrial activations, the post-ventricular atrial refractory period (PVARP) is employed. This timed refractory period begins upon the sensing of a natural R wave or upon a paced R wave. During these refractory periods, the atrial channel is prohibited from sensing in order to preclude sensing far field ventricular activity and creating a false atrial detection. Hence, during this time, the atrial channel cannot initiate the timing of an AV delay for the delivery of a ventricular pace. The PVARP is usually marginally maintained to be short because if set for a long duration it can limit the maximum tracking rate of the pacemaker to be too slow.
Under some conditions, PVARP alone is not adequate to preclude a PMT. A premature ventricular activation, known as a PVC, is the most common cause of PMT. A PVC is a ventricular activation that occurs out of sequence (premature) within a normal intrinsic rhythm without an intervening atrial activation. It occurs earlier than the normal sinus beat and can occur at such a time when the connective tissue between the atria and ventricles can transmit retrograde electrical signals from the ventricles to the atria. When this occurs, a PMT can be initiated even though PVARP may otherwise be adequate.
In view of the foregoing, measures have been taken in the art either to prevent a PMT condition from occurring or to terminate a PMT condition should one occur. One such measure provides for a PVARP extension whenever a PVC occurs. This precludes the need of setting PVARP so long as to adversely limit the upper tracking limit of the pacemaker while affording PMT protection from a PVC initiation. However, this requires heart activity analysis to identify a PVC which adds to the complexity of pacemakers employing this technique. In addition, if a PVC is missed, it does not alone protect against PMT.
A termination measure is the provision of PVARP extension whenever a predetermined number of consecutive ventricular paces have occurred at the pacemaker upper rate. While this does terminate a PMT, it also requires additional complexity for analyzing ventricular pacing trends and exposes the patient to this high rate.
The present invention provides a simple and elegant solution to the PMT problem. As will be seen hereinafter, the present invention prohibits an extended PMT to occur without requiring the complexities of heart activity analysis and ventricular pacing trends as have been utilized in the prior art.